such that the real and imaginary part of \(M(A)\) , i.e. the vector \(\left(Re(M(A)),Im(M(A))\right)\) , is jointly \(S\alpha S\)  with spectral measure \(k(A\times \cdot )\)  for every \(A\in \mathcal {E}_0\)  [1]}.
We refer to \(k\)  as the circular control measure of \(M\) , and denote by \(m(A)=k(A\times S^1)\)  the control measure of \(M\) .
Furthermore, \(M\)  is isotropic if and only if its circular control measure is of the form
\(k=m\cdot \gamma ,\) 
